Biological females are more likely than males to develop adverse events after seasonal flu vaccination, according to a recent meta-analysis of 34,343 patients in 18 randomised controlled trials. 

Compared with aged-matched males, injection site reactions (e.g. pain and redness) were 29 and 43 per cent more common in younger (18-64 years) and older females (65 years and older) respectively.   

Severe injection site reactions were 70 and 51 per cent more common in younger and older females respectively.

Systemic reactions (e.g. head- ache and myalgia) were 25 and 27 per cent more common in younger and older females respectively than males of the same age. 

Severe reactions were twice as common in younger women (relative risk 2.12) and 48 per cent more common in older females. 

The sex difference did not vary between quadrivalent and trivalent vaccines. The authors estimated that there are 115 extra injection site reactions and 74 additional systemic reactions in females compared with males per 1,000 vaccines. 

Most safety outcomes were self-reported and gender may influence health-seeking behaviours and experiences after vaccination. (J Epidemiol Community Health doi:10. 1136/jech-2023-220781)